The present invention relates to an improved current converter for an electrically driven vehicle, in particular an improved inverter.
Inverters are used in electrically driven vehicles to convert the direct current from a rechargeable battery to a polyphase alternating current for driving an electric machine. The principles of electric drives are known to those skilled in the art and do not have to be described further here for the sake of conciseness.
Such current converters typically use three half-bridges, wherein two transistors are connected in series in each half-bridge. The transistors are coupled to one another at one terminal. The other terminals of the transistors are coupled to the positive potential and the negative potential of the current source. A winding of the electric machine is connected to a tap of the half-bridge. The tap of each half-bridge is located between the two series-connected transistors. A freewheeling diode is connected in parallel with each transistor. It is also possible for freewheeling diodes to be connected in parallel with each parallel circuit arrangement.
The application of such inverters in an electrically driven vehicle places high demands on the electronic components. Power transistors, for example MOSFET transistors, IGB transistors or the like, are used as electronic components.
To increase the power of the inverters, a plurality of power transistors are connected in parallel. Power losses arise during operation of the power transistors. On the one hand, power losses arise during the time for which the power transistor is switched on. These losses are caused substantially by the flow of current in a channel region of the transistor and the voltage dropped across said transistor. There are also switching losses, which arise when the transistor is switched on and off. Such current-independent switching losses arise due to the overlapping of a high current density and a high applied voltage at the transistor. The switching losses arise only during very short intervals in switch-on, during the increase in the flow of current, and in switch-off, during the reduction in the flow of current. In the case of an increasing switching frequency and, in particular, in the case of low currents, said switching losses have a substantial proportion of the total power loss.
In the prior art, the parallel-connected transistors are switched simultaneously, that is to say are modulated synchronously with respect to amplitude, frequency and pulse/interpulse ratio.
DE 40 05 168 A1 discloses a switching device for switching on and switching off at a high switching frequency for inverting an electrical variable in connection with a transformer or motor, wherein a driver for switching parallel-connected MOSFET semiconductors and IGBT power semiconductors is provided.
DE 102 50 155 A1 discloses a switched-mode power supply having a main switch and a standby switch to reduce losses in the standby mode.
DE 102 50 154 A1 discloses a switching unit for a switched-mode power supply in which a main switch and a standby switch connected in parallel with one another are provided.
DE 10 2012 013 938 A1 discloses a transmitter head and a system for contactless energy transmission, which has parallel-connected half-bridges, wherein each half-bridge is formed from a series circuit of actuable semiconductor switches.
JP-11150951 discloses an apparatus for modulating the voltage that is output at a load, wherein the apparatus has two parallel-connected modulation bridges controlled by a control module.
JP-07007935 discloses two parallel-connected switches at a rectifier-transducer circuit. First, a second switching transistor is actuated at a comparatively low input voltage. If the output voltage of the circuit exceeds a specific prescribed value, the first switching transistor is actuated.
DE 10 2013 2012 012 A1 relates to a converter having a plurality of parallel-connected central elements, wherein each central element has at least one half-bridge.
DE 693 15 903 T2 discloses that, in a plurality of parallel-connected transistors of a bridge for actuating an electric motor, the transistors are switched on at different times to reduce parasitic effects.
The invention is based on the object of providing an improved current converter, in particular of providing a current converter having a reduced power loss.
This and other objects of the invention are achieved by a current converter, an inverter, and/or an electric drive in accordance with embodiments of the invention.
The current converter according to the invention is configured to convert a DC voltage of a current source to an AC voltage and/or to convert an AC voltage to a DC voltage. The current converter includes a bridge circuit and a control device. The bridge circuit includes a first parallel circuit arrangement coupled to a higher potential of the current source and a second parallel circuit arrangement coupled to a lower potential of the current source.
The bridge circuit includes a plurality of taps, which are each coupled to the first and the second parallel circuit. Each switching element of the first parallel circuit arrangement and the second parallel circuit arrangement includes two line terminals and one control terminal that controls the flow of current from one line terminal to the other line terminal. The control device is configured, in the case of part load, to actuate at least one switching device of a parallel circuit arrangement in such a way that it is not switched on during a cycle. The switching elements of a parallel circuit arrangement are thermally coupled to one another.
A cycle corresponds to a period of an AC voltage signal. The AC voltage signal can be generated by the current converter and can be output at the taps. However, it is also possible for the AC voltage signal to be applied to the taps. Since at least one switching element is not switched on during at least one cycle, the switching losses and hence the power loss can be reduced. Since the switching elements of a parallel circuit arrangement are thermally coupled to one another, a more uniform current distribution across the switching elements can be achieved independently of the operating point.
The switching elements of a parallel circuit arrangement can be thermally coupled to one another in such a way that heat flows from one switching element of a parallel circuit arrangement to another switching element of the parallel circuit arrangement. The flow of heat can cause the temperature difference between two switching elements of the parallel circuit arrangement during operation to be lower than in the case of two separate (i.e., not thermally coupled) switching elements, in one embodiment, for example, to be lower than approximately 5° C., preferably lower than approximately 2.5° C., more preferably lower than approximately 1° C.
The control device can be configured in such a way that, when the current transducer is operated at part load, a greater number of switching elements of the parallel circuit arrangements are actuated in such a way that they are switched on during a cycle, the greater the load is. Since the number of switching elements that are switched on during a cycle depends on the load, the switching losses can be reduced, since as few switching elements are switched on as necessary. Conversely, fewer switching elements are switched on during a cycle, the lower the load is.
The control device can be configured in such a way that, in the event of part load, the number of switching elements of the parallel circuit arrangements that are actuated in such a way that they are switched on during a cycle are selected in such a way that at least one switching device that is switched on during a cycle is located in each case as close as possible to the range of the highest power output or current output. The less switching devices have to be switched on and off during a cycle, the lower the switching losses are. The control device can be configured in such a way that, in the event of part load, the number of switching elements of the parallel circuit arrangements that are actuated in such a way that they are switched on during a cycle is selected in such a way that at least one switching device that is switched on during a cycle is located in each case as close as possible to the range of the highest efficiency, which comprises the highest efficiency.
The range of the highest power output or current output of a switching element when the current transducer is operated at part load can substantially correspond to the range of the maximum power output or current output when the current transducer is operated at full load.
A plurality of the switching elements of the first parallel circuit arrangement or the second parallel circuit arrangement can be arranged on a chip and/or on a substrate. Particularly good thermal coupling is achieved as a result. For example, the substrate can be a ceramic substrate and the transistors can be arranged on the substrate by way of so-called chip-on-board technology (chip on the substrate). However, it is also possible for a plurality of the switching elements of the first parallel circuit arrangement or the second parallel circuit arrangement to be arranged in a common housing without a lower housing. Good heat transmission and thus more uniform current distribution can be achieved as a result.
The current converter can include a half-bridge having a series circuit composed of the first parallel circuit arrangement and the second parallel circuit arrangement, wherein a plurality of the switching elements of the parallel circuit arrangements are located on a chip and/or on a substrate. The series circuit composed of the first parallel circuit arrangement and the second parallel circuit arrangement can be located in a housing without a lower housing. Better thermal coupling between the switching elements can be achieved as a result, which leads to a more uniform division of the flow of current across the switching elements.
The switching elements can have a transistor, a bipolar transistor, a FET transistor, a MOSFET transistor and/or an IGB transistor. It is preferable for all the parallel circuit arrangements to have the same type of switching element.
A freewheeling diode can be connected in parallel with each of the parallel circuit arrangements. The freewheeling diode protects the switching element against currents and voltages from the coil of the electric machine when the switching device is switched off.
The invention also discloses an inverter including the current converter described above.
The invention also relates to an electric drive including the inverter, wherein at least one winding of an electric machine is connected to a tap of the inverter.
Compared to the prior art, while it is in part load operation, the current converter according to the invention switches fewer transistors of each parallel circuit arrangement on and off during a cycle. The power loss can be reduced and the efficiency can be increased as a result. In full load operation, all the transistors of a parallel circuit arrangement are switched on during a cycle. The power loss can therefore be reduced without restricting the functionality. In this exemplary embodiment, the invention further requires just one comparatively small modification of the control device, since the switching frequencies and the switching times are not changed compared to a conventional current converter. In comparison to a conventional current converter, only the number of transistors switched on during a cycle has to be changed depending on the desired amplitude of the alternating current generated.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.